1. Field of the Invention
The present invention relates to power-generator control systems for smoothing an output voltage of a power generator.
2. Description of the Related Art
In an alternator installable in vehicles, when a rotor of the alternator rotates, a field winding wound around a core of the rotor creates magnetic fluxes when a field current is supplied thereto, and the created magnetic fluxes magnetize the core to provide field poles.
The rotation of the filed poles creates magnetic fluxes, and the created magnetic fluxes induce three-phase AC voltages in three-phase stator windings wound around a stator core that faces the rotor core. The three-phase AC voltages are rectified so that a DC voltage is generated as an output voltage from the alternator.
In such an alternator, one phase voltage is different in phase from another phase voltage, and therefore, rectification of three-phase AC voltages induced in the three-phase windings may cause ripple voltages to appear on an output voltage. Cycles of the ripple voltages therefore change depending on the number of revolutions of the alternator (rotor).
In controlling the output voltage of the alternator, it is important to eliminate the ripple voltages appearing on the output voltage of the alternator.
An example of such means for eliminating ripple voltages from an output voltage of an alternator is disclosed in Japanese Unexamined Patent Publication No. H02-51398.
The ripple-voltage eliminating means disclosed in the Japanese Unexamined Patent Publication is provided with a control circuit that controls a field current to be supplied to a field winding of a synchronous generator to thereby regulate a negatively fed-back output voltage of the synchronous generator to a constant value.
Specifically, the control circuit includes a filter. The filter consists of a differential amplifier, a resistor connected between a negative input terminal and an output terminal thereof, a plurality of capacitors, and a switch.
One electrode of each of the individual capacitors is electrically connected with the negative input terminal to which the output voltage of the synchronous generator is input, and the switch is electrically connected with the output terminal of the differential amplifier. The switch works to select any one of the other electrodes of the individual capacitors to be electrically connected with the output terminal of the differential amplifier.
With the configuration of the control circuit, change of a capacitor to be electrically connected with the output terminal of the differential amplifier allows a lagging phase and a leading phase in the feedback loop to be adjusted, thus effectively filtering out ripple voltages from the output voltage of the synchronous generator. This makes it possible to improve the voltage regulation characteristic of the control circuit.
In the structure of the control circuit, the switch is connected with only the other electrode of any one of the capacitors.
The control circuit is normally designed as an IC. In the control IC, the switch is normally composed of a semiconductor switching element(s), such as a transistor.
When the control IC is used under high-temperature environment as a part of a vehicle, a leakage current that flows across region(s) of the semiconductor switching element in which no current should be flowing may be not ignored. This causes the voltage regulation characteristic of the control circuit to deteriorate.
For example, in the control IC with the filter whose differential amplifier consists of a pair of current-mirror transistors, a leakage current may appear to flow from the switch connected with one of the capacitors to the semiconductor substrate of the control IC. The appearance of leakage current may cause a current flowing through one of the current-mirror transistors to be mismatched with one current flowing through the other of the current-mirror transistors. This may cause the ripple-voltage filtering characteristic of the control IC to deteriorate, so some of ripple voltages, which are supposed to be eliminated under no leakage currents appearing in the control IC, remain in the output voltage. This may cause the voltage regulation characteristic of the control circuit to become worse.